In the production of adipic acid by the liquid phase nitric acid oxidation of cyclohexanol and/or cyclohexanone in the presence of a copper-vanadium catalyst there generally results a purge waste stream containing valuable copper and vanadium ions therein of which recovery is desired if the process is to operate economically. These purge waste streams are generally derived from the mother liquor of one or more crystallizations involved in the recovery of the adipic acid and the necessity for purging arises because of the buildup of other dibasic acids such as succinic acid and glutaric acid. It is obvious that in view of the value of such catalyst metals, recovery of them from the purge waste stream is desirable before discarding same.
The basic process for the nitric acid oxidation of cyclohexanol and/or cyclohexanone in the presence of a copper-vanadium catalyst is well known in the prior art. The feed mixture utilized in most industrial processes is a mixture of cyclohexanone and cyclohexanol derived from the air oxidation of cyclohexane although some processes utilize only cyclohexanol or only cyclohexanone as a feed to the nitric acid oxidation. The cyclohexanol and/or cyclohexanone is generally mixed with from 5 to 40 times its weight of an aqueous nitric acid solution, the nitric acid solution being generally of a concentration of 35 to 65% by weight, and there is also added the metal catalyst. Usually the copper and vanadium are added as ammonium metavandate and copper turnings, the total amount of catalyst usually being about 0.05 to 1.0% by weight of the reaction mixture. The liquid phase nitric acid oxidation is generally conducted at temperatures within the range of about 55.degree. to 100.degree.C. and pressures within the range of about 1 to 5 atmospheres absolute.
There is produced in the nitric acid oxidation a liquid reaction product comprising the adipic acid and which also contains succinic acid, glutaric acid, nitric acid, water and the copper and vanadium catalyst values. There is also produced in the nitric acid oxidation an off-gas containing nitrogen oxides such as NO, N.sub.2 O, NO.sub.2 and N.sub.2 O.sub.4 as well as other gasses such as carbon dioxide and nitrogen. Adipic acid crystals are recovered from the liquid reaction product by crystallization techniques, there resulting a mother liquor which comprises an aqueous nitric acid solution containing copper and vanadium values as well as dibasic caboxylic acids (mainly glutaric and succinic acids although some unrecovered adipic acid may also be present). A portion of this mother liquor may be and is generally recycled to the nitric acid oxidation reactor although a portion must be removed or purged to prevent buildup of the succinic and glutaric acid impurities, the portion removed or purged being the above-mentioned purge waste streams. The recovery of the adipic acid by crystallization is well known and such may be accomplished by one or more crystallization stages and may include effecting a removal of some nitric acid and water and re-dilution between crystallization steps.
Various methods have been developed for recovery of the copper and vanadium values from these purge waste streams as may be seen from U.S. Pat. Nos. 3,106,450; 3,186,952; 3,463,740; and 3,554,692. Also see British Patent Specifications 980762 and 956403. The most popular of the methods for the recovery of the catalyst metals from the purge waste streams is by passing these streams through an ion exchange treatment zone whereby the metals are bound to the exchanger and then the metal is recovered by passing nitric acid over the exchanger. The stream resulting from the regeneration of the exchanger consists essentially of a nitric acid solution with the catalyst metals therein and, therefore, can be recycled to the nitric acid oxidation zone. While such a recovery process as well as the various known modifications thereof are fairly efficient in recovering the copper values from the purge waste streams, they do not provide as efficient recovery of the vanadium values as desired.
It is thus an object of the present invention to provide a new and useful process for the recovery of copper and vanadium values from a purge waste stream derived from the production of adipic acid by nitric acid oxidation of cyclohexanol and/or cyclohexanone. It is a particular object of the present invention to provide an improvement in the process wherein copper and vanadium values are recovered by ion exchange means from such a purge waste stream. Additional objects will become apparent from the following description of the present invention.
In the following description and in the claims, all parts and percentages are by weight unless otherwise specified.